This invention relates to Charpy's or falling weight impact test for measuring the toughness of a test material at a cryogenic temperature and, more particularly, to a Charpy's or falling weight impact testing method for measuring the toughness of a test material at a cryogenic temperature and a testing machine for executing the method.
There are heretofore known as conventional testing method of the aforementioned type an impact testing method for testing the toughness of a test material at a cryogenic temperature having the steps of immersing the test material in a liquefied gas (such as, for example, alcohol, Freon or the like) cooled and retained at a low temperature, maintaining it for a predetermined time, rapidly moving the test material from the liquefied gas, setting the material in a testing machine and testing it; and an impact testing method for testing the toughness of a test material at a cryogenic temperature having the steps of enclosing the test material with a heat insulating material such as, for example, a foamed styrol, paper or the like formed in a box, setting the box in a testing machine, filling low temperature liquefied gas such as, for example, liquefied helium or the like therein, and testing the material in this state.
The specific heat of a substance generally becomes low at a low temperature, and when the temperature falls at a temperature particularly to that of liquefied gas, the specific heat of the substance becomes extremely low.
When a test material or piece of aluminum alloy is, for example, removed from liquefied helium to the air, it is reported that it is raised at temperature of 60.degree. C. for 1.5 to 2 seconds according to its observation.
The former testing method of the above described ones has such disadvantages that, since the test material is exposed with the air after it is cooled, its temperature instantaneously rises and becomes unstable to cause the test at the set low temperature to become difficult and to also cause the measured value to lack its reliability.
On the other hand, the latter testing method of the above described ones has such disadvantages that, though it can eliminate the aforementioned disadvantages because the test material may not be exposed with the air, since the heat insulating box is destroyed at every test, it is not only inefficient but also troublesome and complicated because it must correct the effect of the heat insulating box to the measured value of the strength and it is difficult to control an arbitrary temperature in the method.